Photodynamic therapy (PDT), using hematoporphyrin derivative (HpD) or its more purified form, di-hematoporphyrin-ether (DHE), is presently undergoing extensive clinical trials. PDT is based on the selective retention of HpD/DHE by neoplastic tissue and the ability of it to be photodynamically active. The therapeutic response obtained is a product of the HpD/DHE present in the tissue and the light it absorbs. Physiological factors such as tissue O2 content, ph and temperature may also play a role in the therapeutic response. It is proposed in this Phase-I study to develop a prototype single optical fiber probe that can measure (1) the light space irradiance in-vivo, (2) the fluorescence of the HpD/DHE in-vivo, and (3) the absorption dose rate and therapeutic dose in-vivo. Such a probe would be useful in monitoring the photodynamic reaction in-vivo and thereby provide for a more optimum treatment. The system would consist of a single quartz fiber for delivery of the fluorescence exciting light and to pick up the fluorescence signal. A rationing of selective wavelengths in the fluorescence emission will permit separation of HpD/DHE fluorescence from other background fluorescence. In Phase-II extensive animal and pulmonary clinical trials will be undertaken, along with further refinement of the instrumentation.